Infected Windows PCs display a dialogue box asking for the unlock code and the hint that they can find it after visiting a website linking to a list of different prize surveys or by downloading unnecessary software such as a media player.

The box won't clear until the survey code has been entered, and can;t be closed using the task manager; attempts to delve into matters using the command prompt, PowerShell, Regedit, or MSConfig are also denied as is the ability to bypass it by invoking a restore point.

Entering the code incorrectly three times, or just attempting to close the dialogue, causes the system to shut down. Upon a reboot the same dialogue reappears after 20 seconds, the length of time the users have to try and shut it down using the Task Manager.

Shadowlock can also nix browsers and certain system tools as well as consume free resources and disable the Windows firewall.

Built using .NET, Symantec was able to decompile it well enough to discover some of its more eccentric secrets, including an 'Easter egg', a hidden routine that plays a the five-note theme from the 1976 alien abduction film Close Encounters of the Third Kind.

Other abilities include being able to reverse mouse buttons and open the CD tray or open Windows utilities.

"It turns out the malware author has a sense of humor," wrote Symantec researcher, Fred Gutierrez in his blog on Shadowlock.

He speculates that the survey tactic might be an experiment to see much response it gets, or perhaps part of a genuine money-making scheme.

"These functions (as well as others) may find themselves being used in a future variant."